impact of coal on cases with lots of unused capacity?

[evtSmtx]

Does anyone have any data at all(even subjective) on loading at COAL longer/shorter than recommended?

For example .45 ACP 230gr fmj in front of 4.7 grains of titegroup has oodles of empty space. Would I get any more velocity/less soot from shortening down from 1.25"?

 

[BCRider]

I do not consider any of the short semi auto cases as being a large enough internal volume that you can ignore anything other than very small changes to the listed COAL in the loading data.

And remember that what you see when you look into the case isn't what is left after the bullet is seated. That volume goes WAY down when it's filled up with the backside of a bullet.

When chrono testing 9mm ammo using the same charge weight I found that COAL variations of .010" produced roughly 40 fps changes in velocity. I consider that amount to be significant. Especially when I was doing this with light 130PF loads intended for my IPSC shooting. Because of the way the powder burns under pressure you can expect more effect from the same change when you alter the COAL with near max charges.

On the other hand by comparison the VASTLY larger internal volume left by light charges inside longer revolver cases such as .38Spl and .357Mag compared to 9mm and .45Colt compared to your .45ACP means that a .010 variation wouldn't even be a drop in the bucket. In that case (pun not intended ) bullet weight becomes the dominant effect on pressure for a given charge weight. But for the short semi auto cases I'd say that while bullet weight is still the major player you can't discount the bullet seating effect. Especially if you're working near the max load for the powder.

Because of this I don't even bother recording my COAL for revolver loads other than my .357Mag and .44Mag loads. I either just load to the cannelure groove when one is present or to where smooth sided jacketed bullets look more or less the same as the loads using cannelure grooves. But for my semi auto rounds I pay attention to the COAL given in the load data and if I vary from it I take care to consider the amount of powder and the change it'll have on the peak pressure. Or where possible I'll also look for data that lists COAL's closer to what I need to use for any particular gun.

My CZ guns are notable for this since both need COAL's that are on the short side or the rounds will jam against the rifling before seating correctly.

 

[Sunray]

The amount of 'soot' produced has nothing to do with the OAL. The amount of 'soot' has to do with how cleanly the powder burns.
OAL for a 230 is 1.200" too.

 

[BCRider]

The soot would be reduced only in proportion to the amount that it raises the pressure and holds the pressure for longer. And while I feel that the 40fps from an .010" change is significant enough to care about it isn't enough to make any difference in soot blow back in real terms.

Real change in soot blow back between the cases and chamber is going to come from running closer to full power charges and using powders known for less soot production.

 

[TimSr]

Would I get any more velocity/less soot from shortening down from 1.25"?

Yes, and yes, but you will be driving up the pressure that will give you those results so not a good way to go unless you have published pressure data for deeper seating.

By the way, seating depth is your concern, not so much OAL which can change with the shape of the bullet.

 

[gamestalker]

There is no risk if you first take the charge down and then re-work it at a shorter OAL. It's when we shorten the oal on an already developed charge that pressures can get dicey. The only real problem one might experience is feeding issues, so seat em where you want them, as long as they will feed reliably.

GS

 

[Rule3]

Why would you load COAL in you cases?

The COL is there for a reason, that is why they test powders bullets and the seating depth or (COL) to determine safe pressures. Yes you can fudge a little to fit your barrel but increasing seating depth is not directly related to reduction in powder so it's just guess work.

Use the recommended COL and do not seat deeper the the minimum.

 

[grogetr]

I read in a Speer manual they shortened the oal by .030" in a 9mm and it almost doubled the preasure. Be careful.

 

[rsrocket1]

Here's a hypothetical "work up" showing pressure and MV versus COL with 9mm I ran with Quickload a while ago.



While the MV goes up, the Pmax goes up faster so you can't really predict where the Pmax will get to a dangerous level by using a chrony.

As TimSr said, you'll get higher velociy and a cleaner burn (provided the "dirty" burn was from too low a pressure with your original load). It is much safer to work up loads with more powder so long as you stay below the max published load.